Japanese Patent Application Publication No. 2001-351789 (FIG. 1) (hereinafter referred to as JP2001-351789) discloses a technique of dimming an LED load by connecting the LED load to an output of a half-bridge inverter circuit via an LC series resonant circuit and varying a switching frequency.
Japanese Patent No. 2,975,029 (FIG. 5) discloses a technique of dimming a discharge lamp load by connecting a hot cathode type discharge lamp load to an output of a half-bridge inverter circuit via an LC series resonant circuit and setting ON periods of two switching elements of the inverter circuit to be unequal during dimming. Further, there has been proposed a technique of supplying a preheating current while avoiding cold cathode discharge by setting the ON periods of the two switching elements of the inverter circuit to be substantially equal during preheating and setting a switching frequency to be sufficiently higher than a resonant frequency to reduce a resonant voltage applied to the load.
In accordance with the technique of JP2001-351789, the dimming operation of the LED load is performed by varying the switching frequency. Thus, in order to widen a dimming range, it is necessary to expand a variation range of the switching frequency, and there is a problem in that a high frequency side switching loss increases, or it is difficult to design a filter circuit for removing a switching noise. Further, the LED load has diode type load characteristics in which the load current hardly flows therethrough when a voltage across the LED load is equal to or less than a predetermined load voltage. Accordingly, in case of increasing the switching frequency, the resonant voltage applied to the load is reduced, and there is problem in that it is impossible to obtain a voltage required for turning on the LED load.
In JP2001-351789, there has also been proposed the technique of expanding the dimming range by intermittently pausing a high frequency switching operation at a low frequency (see Paragraph [0099] and FIG. 15 in JP2001-351789). However, in such case, there is a problem of an increase in flicker.